go to forest area 17 and only fish the biggest fish. there are something like 5 different fish with some being easier than others, that are in the whopper category. you need to have some different hooks unlocked through sidequests though. you can find them in the forest base camp.
these are the whopper class fish. i don't know if there are more, but these are what i found in forest 17
Great Trevally, Gajau, Speartuna, Gastronome Tune and Goliath Squid
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Great Trevally, Gajau, Speartuna, Gastronome Tune and Goliath Squid